This invention relates to an improved, simple method and apparatus (portable) for a direct digital display of the electrical conductance of body fluids, specifically, the hematocrit determination of whole blood specimens.
There are a variety of existing methods and apparatus used for hematocrit determinations but no prior method and/or apparatus exists which determines the hematocrit of blood with a portable, digital display form which only requires diping the electrode-probe into a test tube, or by only placing one drop of blood onto the electrode surfaces, and within 10 seconds have high accuracy, reproducibility and still maintain simplicity of use and ease of electrode cleaning.
Prior art techniques include the classical hematocrit measurement method by centrifugation of blood samples contained in capillary action tubes and then visually measure the relative packed cell height to the total blood sample height. This time consuming method has disadvantages which are well known to those skilled in the art; specifically, the inaccuracies due to technician errors resulting from inadequate sample centrifugation time and visual interpretation.
A second technique computes the hematocrit value from the product of the mean cell volume and the red cell number. By measuring the average population size of the red cells the mean cell volume is determined and simultaneously count the number of red cells diluted in a specific volume of fluid. An electrical conductance technique is utilized to evaluate the mean cell volume and the red cell number; it may give spurious results because of dilution errors and errors due to counting particles other than red cells, i.e., white cells or more than one cell at a time, such as the doublets or triplets.
Other systems measuring the hematocrit by means of electrical conductivity of whole blood may suffer from the inherent errors of bridge balancing circuitry, electrodes which are cumbersome to use and/or difficult to clean; other systems may have errors due to inadequate sample temperature compensation, inadequate polarization and reactance compensation and no non-linear compensation.
This invention, therefore, has the following objects to provide for an improved method and portable apparatus for the direct digital display and simplified measurement of the whole blood hematocrit. This technique provides two types of simple to use and easy to clean electrode probe assemblies which probes are to be used by either: (1) diping the probe directly into a test tube filled with anti-coagulated blood specimen, or: (2) placing one drop of blood onto its surface, covering the surface and the blood with a cover slip and measuring the hematocrit instantly.
Provisions are made whereby electrode polarization tendencies are eliminated and furthermore the sample temperature from 68.degree. F. to 104.degree. F. is compensated for immediately by the thermistor element which is included within the probe assembly.
A further object is to provide a solution to the Fricke, Curtis and Maxwell equations relating the whole blood conductivity to the hematocrit of the sample by means of an exponential approximation to the said equation.
Another object provides for maintaining uniformity between individual observers and to alert said observers automatically of insufficient battery power and/or poor solution-electrode contact.
Also a provision is made for obtaining a hemoglobin estimate as derived from the measured hematocrit value assuming the sample to have normal cell indices.